Sydney's iconic "red rattler" trains were finally pensioned off more than 20 years ago, and ever since the city's passenger railway network has been dominated by double-decker carriages.

But NSW Premier Barry O'Farrell says that was the wrong call by transport planners.

"Single-deckers... can carry more people, travel more quickly, and disembark those people more quickly without people having to come down from those difficult steps that exist on our double deckers and that delay people at railway stations," Mr O'Farrell said.

Single-deck trains will be returning to the Sydney rail network in the next five years with the North West Rail Link. That $8.3 billion project has created controversy because double-deck carriages will not be able to run on the new line.

ABC Fact Check asked Mr O'Farrell's office if he meant to imply individual single-deck trains can carry more passengers than a double. A spokesman said he did not; rather that single-deckers can carry more passengers per hour overall.

The claim: Barry O'Farrell says single-decker trains can carry more people than double-decker ones because people can move on and off more quickly.

The verdict: Passengers can move on and off single-decker trains more quickly than double-decker ones, but experts say double-deckers carry so many more passengers that, per hour, they outdo single-deckers.

The North West Rail Link

Due for completion by the end of 2019, the new line between Chatswood on Sydney's north shore and Rouse Hill in the north-west will require 15 km twin tunnels to be dug.

The project's website says those tunnels will be six metres in diameter, large enough to carry the new generation of single-deck trains, but not the existing Sydney double-deckers.

A June 2012 project overview said: "Single-deck trains have the advantage of being able to load and unload quickly at stations, allowing more trains per hour on any given line."

The tunnelling and station excavation contract is worth $1.15 billion, according to a report from the NSW Government agency Transport for NSW. Enlarging tunnels and stations for double-deckers would cost $200 million more than for singles. The same document says a double carriage costs $1 million more than a single one.

The project's website says the new single-deckers will be driverless and automated "metro" style trains, with sliding screen doors on platforms for safety.

The NSW Government identified some of the features of the trains in its report Sydney's Rail Future,including:

Three or more doors per side per carriage (compared to two doors on exisiting double-deckers)

Level access between platform and train

A mixture of seating areas

The configuration of the carriages will be finalised following procurement, however the project overview calls for single-deck trains capable of moving up to 1,300 people.

At peak times, the website says, trains will run at least every five minutes - a rate of 12 services per hour. A Transport for NSW spokesman says the new line will have the capacity to increase to up to 20 trains an hour as demand grows. On that basis, the new line could move between 15,600 and 26,000 passengers per hour.

The full journey from Cudgegong Rd at Rouse Hill to Chatswood is expected to take 37 minutes. Passengers will then be required to change trains at Chatswood from the single-deckers to double-deckers to continue their journey into the city and beyond.

Sydney's double-deckers

Double-deck trains began appearing on the Sydney suburban passenger rail network 50 years ago to increase capacity without the need for major engineering works.

Although described as double, they have always been a hybrid with a single deck at each end near the doors, and a double "gondola" in the middle between the bogies. The 1964-vintage Tulloch double deckers effectively doubled the capacity of each train.

The Tulloch carriages had seating for 132 passengers and standing room for 146 more, compared to 70 seats and around 60 standing on the earlier single-deck C-class red rattlers also known colloquially as "Sputniks". The last of the old-style single-deck trains were retired in the early 1990s.

The double-deckers on the Sydney rail network now carry about a million passengers each weekday. They frequently run at a rate of up to 15 services per hour.

Comparing capacity

The newest trains running in Sydney are the double-decked Waratahs, whose eight-car trains have a seating capacity of around 900 and according to Sydney Trains a total "reliable capacity" of 1,210 seated and standing.

But it is possible to pack many more people into a Sydney train than that. Tests by RailCorp in 2007 found the maximum practical load of an eight-car double-deck train is 1,750, but only if it is a single destination special such as for a major sporting ground where all passengers get on and off at once.

Double-decked trains overseas such as the MI09, which entered service with the Paris RER Line A in 2012, can carry up to 1,725 passengers in just five carriages. This translates to a notional eight-car capacity of 2,760.

The capacity of single-deckers has increased since the days of the red rattlers, with many now favouring seats along the walls and increased standing room in the middle rather than rows of seats across the carriage.

A look at interstate and international trains suggests the North West Rail Link target capacity of 1,300 per single deck train is realistic if a high proportion of passengers stand. Queensland Rail's SMU and EMU fleet are single deck, and carry up to 500 passengers in a three-carriage train (240 seated and 260 standing), giving a notional eight-car capacity of around 1,330.

Hong Kong's Mass Transit Railway boasts a total capacity of 2,500 passengers on its single-deck eight-car trains on the Tsuen Wan Line, with no separate data available on the number of those who can be seated.

Estimates contained in a report by Infrastructure NSW state double-deck and "single-deck comfortable" trains both have a total capacity of 1,200, with 890 seats on the double-decker and 600 on the single.

Given the almost unlimited variations between carriage size and design, a direct capacity comparison between doubles and singles is problematic.

However, transport economist Neil Douglas, who has conducted extensive research for the NSW Government, puts the practical capacity at 1,400 for doubles and 1,120 for singles.

Train dwell times

Mr O'Farrell says people can disembark more quickly from single-deck trains than doubles.

A range of factors can extend "dwell time", the time a train must wait at a platform for passengers to get off and on. These include the number of doors and stairs on the train and crowding levels in the carriage or on the platform.

According to data compiled for the NSW Government by the engineering firm Parsons Brinckerhoff, double-decker dwell times at Sydney stations range from 35 seconds at suburban stations to 70 seconds in the central business district. By way of comparison the single-deck Dubai Metro, which claims to be the world's longest automated train service, keeps boarding times at stations down to an average of just 30 seconds.

Parsons Brinckerhoff modelling, quoted in a report from Dr Douglas's Douglas Economics, assumes single-deck trains have a dwell time 10 seconds less than double-deckers at the busiest stations in Sydney.

Using the Douglas Economics capacity estimates, the North West Rail Link would carry between 13,440 passengers on the 12 trains per hour specified by the project overview, and an eventual 22,400 on the 20 services predicted by Transport for NSW.

That gives doubles a 5,600 passenger per hour advantage at 20 trains an hour, which is equal to another five single-decker trains carrying 1,120 passengers per hour.

However, according to his research, Dr Douglas says it would only be possible to run an additional two single deck trains per hour, resulting in a peak load of 25,000 passengers per hour.

The impact of new technology

The Infrastructure NSW report says the new "metro" style of driverless single-deck train has a maximum load of 2,000 passengers with 400 seated and 1,600 standing.

The report states conventional single-deck and automated single-deck "metro" trains are both capable of running 30 services per hour, while it says double-deckers can only run 20.

However that difference of 50 per cent cannot be explained by a relatively short dwell time difference of around 10 seconds per stop.

The report says those assumptions also depend on "the introduction of new train control systems, using technology that is proven in service overseas".

Dr Douglas says the 20-per hour limit on double deckers is due to signalling, which only allows for a maximum of one train every three minutes, and equally applies to single-deckers.

Improvements to signalling would allow 24 double-deck trains per hour carrying a peak load of 33,000 passengers per hour. Alternately, it would also allow for 26 single deck services an hour, with a peak load of 29,200 passengers.

Double-deck v single-deck train capacity, peak load per hour

Existing signalling

Enhanced signalling

Double-deck

20 trains, 28,000 passengers

24 trains, 33,600 passengers

Single-deck

22 trains, 25,000 passengers

26 trains, 29,200 passengers

Source: Douglas Economics

Equally, other improvements foreseen in the new "metro" single deckers can also apply to double-deckers.

Dr John Stone, who lectures in urban planning at the University of Melbourne, says double-deckers could be made faster. "Double-deckers don't need to cause long dwell times," he said. "Platform signs that show where doors will be and the overall sense of the importance of speed allow double-deckers to work well in Europe."

Double-deckers such as the Paris MI09 have added a third pair of doors in each carriage to speed up the boarding process and reduce dwell times.

In theory, according to Dr Douglas, double-deckers could also be automated and achieve comparable time savings. "I can see no reason why future double-deckers could not operate in exactly the same way as a single-decker - driverless or not," he said.

Experts weigh the merits

Professor Graham Currie from the Institute of Transport Studies at Melbourne's Monash University says that the shorter dwell time of single-deckers only partly counteracts the benefit of the larger capacity double-deck cars.

Dr Phillip Laird, a rail transport expert at the University of Wollongong, says double-deckers are better for long distances, while single-deckers are better for short trips in the CBD and to the airport.

And Dr Stone agrees it isn't an either-or proposition. "The Sydney rail system has need for both for different trip types," he said.

However, Dr Douglas cautions passengers simply won't want to spend a 37 minute journey standing up and that a lack of seats will lead to lower passenger demand on the North West Rail Link.

Dr Laird also says the new line would be better served by double-deckers given the distance from the city.

The verdict

Given their smaller passenger capacity, single-deck trains can only transport more passengers than doubles by running significantly more services per hour. However, the dwell time saving of single-deck trains is relatively slight, by one estimate only allowing for two additional services per hour.

Any other time savings are derived from automation or improved signalling, which can be applied equally to singles or doubles.

Unlike the red rattlers, newer singles often boost their capacity by having most passengers travel standing. While this raises concerns about passenger comfort on longer trips, the same decision to reduce the number of seats could also apply to double-deckers, which would still have greater capacity.